the water. Such, too, is the effect of refraction on rays of light; which, on passing into water, or any heavier transparent body, are bent downward.

Obs. To verify and understand this principle, put a halfpenny into an empty basin, and stand at such distance that the coin may not be visible; then, let another personr pour pure water into the basin, and the halfpenny will become visible: this arises from the bending of the rays in their passage into air at the surface of the water.

576. Hence, when the rays of light coming from the celestial bodies, arrive at our atmosphere, they are bent downward; and those bodies appear, when in the horizon, a degree higher than they are.

Many rays of light are reflected at the surface of a new transparent medium, in an angle equal to that in which they fall on the surface, and on this principle all mirrors are constructed.

577. Advantage is taken by man of the property of refraction, to construct new mediums of such shape, as that all rays that fall on them may, on coming out of them, converge in one point instead of going straight forward.

The construction of surface which produces this effect is the convex; and all rays of light which fall on a circular surface of glass, &c., are converged on the other side into a series of corresponding points, representing the objects whence the light proceeded.

Such are spectacle-glasses, called lenses.

Obs. Two or three lenses, of different degrees of convexity, may be purchased at any optician's for sixpence

each and the various illustrative experiments that may be tried with them, render it desirable that they should be at hand, in every seminary of education.

578. The eye consists of a transparent horny coat on its outside, called the cornea; within it, is a pure liquid called the aqueous humour; and within the aqueous humour, is a lens, like a spectacle-glass, called the crystalline humour,

Beyond that, is a jelly-like humour, called the vitreous humour, filling the ball of the eye; and at the back of the eye, is spread the optic nerve, retina, or fine net-work, to receive the impressions of the rays of light.

The horny coat, the lens, called the crystalline humour, and the other transparent humours, answer the general purpose of one spectacle-glass, with nice and wonderful powers of adaptation.

The Circle represents the ball of the eye, and the arrow ABC, any object to be seen by the eye, Rays proceed from all points of the arrow, but some of those from ABC only, are represented, to avoid confusion, These flow from A B C in straight lines as represented, They fall on the cornea at E F, pass through the aqueous humour to the iris or pupil; thence, through the convex lens or crystalline humour, shaded dark, and thence, into the

ball of the eye, filled with the vitreous humour, reconverging and producing a perfect picture of the arrow at the back of the eye at c, b, a, where is spread the fine net-work, or retina of the optic nerve. The tube at the corner, is the optic nerve going to the brain:-such is the simple, but wonderful economy of vision!

Obs. To comprehend the effect on the lens of the eye in producing vision in the different ways in which the rays may fall upon it, hold a spectacle-glass, by means of a rule or stick, at unequal distances from a wall; which wall may be supposed to represent the back of the eye. Then place a candle at such distance, or adapt the lens to that distance, and a beautiful picture of the candle reversed will be seen upon the wall. Keep the lens fixed, and move the candle nearer to it, and the image will be less distinct, and quite vanish, as it approaches; then carry the candle backward, to a greater distance than the first distance, and, in like manner, the image will again become indistinct. In the first instance, the rays fall with such a degree of obliquity, as, when operated upon by the refraction of the glass, oc casions the whole of them to converge, and reproduce, in opposite and cross directions, an image of every part of the candle; but when carried nearer, the refracting power of the glass is unequal to the great degree of convergency required, and either the image will be produced at a greater distance, that is, beyond the wall, or the rays will go out parallel; or even diverge or spread, if the candle be carried till nearer. Hence, vision depends on the parallelism or obliquity of the rays proceeding from an object, and on the power of the eye to accommodate itself to that obliquity; and when that is greater than the power, art is necessary to diminish or increase the obliquity of the rays, so as to accommodate them to the powers of the eye. Aged people require spectacles to increase the convergerty, because in these, all the humours diminish, and the chrystalline lens of the eye becomes flatter, and the cornea itselfless convex: hence, the power of convergency is diminished, and the images of objects fall beyond or behind the optic nerves. This can be illustrated in a moment, by having two lenses of different convexities held at the same dis

fance from the wall; and it will be proved, that whea the more convex, or youthful lens, produces a distinct picture, the flatter, or aged one, produces a confused image.

579. The different distances, at which lenses produce on a wall the representation of objects, is called their focal distance; and it is the centre of the circle, of which the surface of the common double lens is a part.

The concave lens has the opposite effect; it diverges or spreads the rays, instead of converging them to a focus.

Hence, when the eye is too flat in old age, the convex lens' helps its converging powers.

And when it is too convex, as in short-sighted people, the concave lens counteracts the convexity of the eye, spreads the rays, and renders Vision distinct.

Obs. 1,--The next circle represents the ball of an aged eye; in which, owing to the decay of the humours, the cornea is not convex enough to converge the rays on the retina, but only a little beyond it. The object wilf, therefore, appear with a burr around it, or confused, If, then, a conner lens or spectacle glass be interposed, as a, b, this will give a converging direction to the pack before they reach the eye; and, of course, its converging power will then be sufficient to produce the figure exactly on the optic nerve,

THE CONVEX LENS FOR AGED PERSONS,

Obs. 9. In this figure, the cornea is supposed to be too convex, as in short-sighted persons; and the rays are converged before they reach the back of the eye or retina. The eye, in this case, performs too much, and more is given it to perform, by interposing a concave lens, so that the rays, instead of falling parallel on the eye, may actually fall divergent.

THE CONCAVE LENS FOR SHORT-SIGHTED PERIONS.

580. But besides the useful invention of spectacles, arising from this power of converging rays of light in convex lenses, a very important use arises from their combination in microscopes and telescopes, the principle of which is exceedingly simple.

Every visible object is of a visible size, proportioned to the angle which it makes to the eye; and that angle is also always in the inverse proportion of the distance, which the eye is from the object.

Obs. To prove this, try a simple microscope without any glass, and it will enable you to see an object clearly at the distance of an inch; which, with the naked eye, could not be well seen at less than eight inches; and the

The term inverse signifies something like a contrary. Thus the size is not as the distance; because, as the distance is greater, the size is less; the proportion is, therefore, not direct, but opposite, contrary, or inverse.